Method of manufacturing reflectors for a solar concentrator apparatus

ABSTRACT

Glass reflectors for concentrating sunlight in a solar energy system are disclosed. A concave mold is used to shape the glass reflectors in which the surface of the mold that contacts the float glass has a grooved surface profile comprising a plurality of cusps. The surface of the mold also has a plurality of concave valleys. Glass reflectors are preferably manufactured by heating a sheet of float glass positioned over the mold until the sheet of glass sags to contact the mold. If necessary, the glass may also stretch to conform to the shape of the mold. In the case of large monolithic glass reflectors, the edges of the dish-shaped glass are rolled around the periphery of the mold. The glass reflector is then silvered to create a dish-shaped mirror that reflects solar radiation to a focus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/463,026 filed May 8, 2009, entitled “Method of ManufacturingLarge Dish Reflectors for a Solar Concentrator Apparatus,” now U.S. Pat.No. 8,082,755 issued Dec. 27, 2011, the entire disclosure of which isincorporated herein by reference. The '026 application is based upon andclaims the benefit of the filing date of U.S. Provisional PatentApplication Ser. No. 61/127,298, filed May 12, 2008, entitled “SolarPhotovoltaic Generator,” the entire disclosure of which is incorporatedherein by reference. This application is also a continuation of U.S.patent application Ser. No. 12/463,016, filed May 8, 2009, entitled“Photovoltaic Generator with a Spherical Imaging Lens for Use with aParaboloidal Solar Reflector,” by Roger P. Angel, now U.S. Pat. No.8,350,145 issued Jan. 8, 2013, the entire disclosure of which isincorporated herein by reference. This application is also acontinuation of U.S. patent application Ser. No. 12/463,001, filed May8, 2009 entitled “Solar Concentrator Apparatus with Large, Multiple,Co-Axial Dish Reflectors,” by Roger P. Angel and Warren B. Davison, nowU.S. Pat. No. 8,430,090 issued Apr. 30, 2013, the entire disclosure ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Climate change is recognized as a significant problem that has receivedconsiderable attention. As a result of worldwide generation of energyfrom fossil fuels, large amounts of greenhouse gasses are accumulatingin our atmosphere. Many experts believe that if something is not donesoon to slow or even reverse this accumulation, our climate and theworld in which we live will suffer catastrophic consequences. Expertspredict that a rise in global temperatures of just a few degrees willmelt polar ice, and result in a rise of sea levels enough to put manycoastal cities under water. The extinction of many species of plants andanimals is also predicted by some scientists. In view of these and othersignificant adverse effects from burning fossil fuels to generateenergy, there is a significant need for a method and apparatus that cangenerate energy in a cost-effective manner without the generation ofsignificant greenhouse gasses.

The present invention is directed to facilitating conversion of solarenergy into useful electrical or chemical energy by processes thateither require strongly concentrated solar energy, or are made moreefficient or more cost effective with concentrated sunlight. Inparticular, the present invention is directed to an apparatus forelectricity generation through photovoltaic or thermal processes. It mayalso be used for converting solar energy to chemical energy by thermalor photochemical reactions.

The efficiency of thermal conversion is limited by the second law ofthermodynamics, which requires high temperature and hence highlyconcentrated light for high conversion efficiency. Photovoltaicconversion efficiency may also be improved in highly concentrated light.Thus, in the past, significant effort and attention has been given tothe problem of converting solar energy at high concentration, but theresults have not been altogether satisfactory. Despite the availabilityof solar conversion devices that operate more efficiently in highlyconcentrated light, high concentration solar energy systems have not yetbecome a major factor in global solar electricity production. Asignificant drawback to prior attempts at high concentration systems hasbeen the high cost of the opto-mechanical systems required to providehighly concentrated light to solar conversion devices. Prior effortshave not been cost competitive, in part, because of the unfavorablescale sizes of the optical concentrators. These prior attempts ofteninvolved the use of particular conversion devices requiring input ofconcentrated sunlight at either very low levels or very high levels ofpower, and such power requirements led for the most part to either verysmall or very large optical apertures for the sunlight concentrators.This design approach failed to adequately minimize the cost per unit ofgenerated electricity (or other form of energy).

In the past, small unit size was favored for most photovoltaicconverters. This led to inefficiencies and high costs. Arrays of largenumbers of small units with small optical concentrators were expensiveto manufacture, assemble and transport, because they were both large andcomplex, with many small optical and electrical and thermal componentsarrayed over a large area. Also arrays of small units conferred littlestiffness on large scales, and would be carried as dead weight on atwo-axis solar tracker. Substantial additional weight in the form ofstructural members was needed to control gravity and wind bending and tomake connection with the tracking mount. This design approach addedsignificantly to the cost of a tracker to be used in such systems, andfailed to achieve adequately minimized cost per unit of generatedelectricity (or other form of energy).

In the past, very large unit size was favored for solar thermal systemsrequiring very high solar power input at high concentration. Largeoptics concentrating in two dimensions to meet these requirements led toinefficiencies. In some systems a thermal converter unit was mounted atthe focus of a single large dish, with a dish-engine assembly carried bya two-axis tracker. In other systems, a thermal converter unit was fixedon a tower with sunlight concentrated on it by a field of typicallythousands of flat mirrors on heliostats. Both of these design approacheswere sub-optimal in terms of minimizing the concentrator system cost perwatt delivered, and failed to adequately minimize the cost per unit ofgenerated electricity (or other form of energy).

For large single dishes, structural complexity and both mass and costper unit area all rise with dish area. In the past, attempts were madeto use a large reflector assembled from many small curved reflectorsegments. In a large reflector made from numerous curved reflectorsegments, each individual segment must be aligned on a paraboloidalback-up truss structure. A composite large reflector assembled in thismanner tended to be heavy and expensive to manufacture and assemble. Inmany cases, large round dishes were mounted individually on two-axistrackers and set out in a large solar farms. Such large round dishesmust be spaced well apart to avoid significant self shadowing in theearly morning and late afternoon, resulting in sub-optimal use of land.A further disadvantage of large round dishes was relatively high windload per unit area, which required heavier and more expensive supportstructure to resist wind loading.

In the past, very high power concentration was attempted by using afield of heliostats. A significant disadvantage to this approach was theinefficient use of trackers carrying a given reflector area. Thisinefficiency resulted because the sunlight incident on many of theheliostat mirrors was at angles far from normal incidence, thus thedaily average of solar power delivered to the power tower was only afraction of what could be captured if each mirror could be tracked toface the sun. Another disadvantage was the mechanical complexity of manysmall two-axis trackers for heliostats.

A further difficulty with many prior solar concentrator systems has beenthe special demand placed on their two-axis trackers. Compactaltitude-over-azimuth mounts on a vertical pedestal have been used.Typical designs were not balanced about the elevation axis, andtherefore typically suffered from high, concentrated drive loads andconsequently required heavy drive mechanics. Of course, heavy drivemechanics increased the cost of such systems, and in solar energysystems, cost is a critical factor that separates success from failure.Large, high concentration dishes for thermal conversion were oftenespecially compromised in their mechanical structure by a large radialopening for an arm to support a fluid heater, engine or turbine andelectromagnetic generator at the focus.

Prior systems for using concentrated sunlight have left significant roomfor improvement. Solar energy systems are unlikely to have a significantimpact on reducing greenhouse gasses in the atmosphere until electricitycan be generated using solar energy at a cost that is competitive withelectricity generated by burning fossil fuels. Cost is critical to solarenergy systems. In fact, cost cannot be over emphasized, because it isso important that cost alone can make the difference between success andfailure. As long as solar generated electricity costs more thanelectricity generated by burning fossil fuels, there is little chancethat solar power is going to have a significant impact on reducinggreenhouse gasses in our atmosphere. There has been a long felt need foran apparatus and method of manufacture for a solar conversion systemthat has a low total system cost and that is capable of generatingelectricity at a cost that is competitive with electricity generated byburning fossil fuels.

OBJECTS AND FEATURES OF THE INVENTION

A central objective of the present invention involves providing a methodof manufacture and an apparatus for generating electricity or otherforms of solar power at low cost. Cost is of criticalimportance—otherwise solar conversion systems are unlikely to displace alarge percentage of conventional electrical generation plants thatgenerate electricity by burning fossil fuels. The present inventionincludes a method of manufacture that facilitates a low total systemcost for solar conversion.

The present invention involves an apparatus to deliver highlyconcentrated solar radiation to high efficiency conversion units at thelowest cost per unit power. To achieve this goal, in the designoptimization process, total solar radiation delivered to each individualunit in a solar conversion system was taken as a variable parameter, andthe aperture of the unit optical collection system and the associatedmechanical structure was varied to find the minimum cost per watt ofgenerated electricity. By establishing the power level for minimum costper watt, highly efficient conversion units could be re-optimized forthis power level, with little or no loss in efficiency, or increase inconversion cost per unit power. For example, in the case of concentratorphotovoltaic cells used in accordance with the present invention, inputpower concentrated into unit levels above about 50 W may be accommodatedin a conversion unit that incorporates multiple, densely packed cellswith active cooling, and provides for equal illumination of all thecells. In addition to being useful in systems employing photovoltaiccells, the present invention also has the advantage of providing lowcost input power for optimized thermal and chemical conversion units.

In order to have a discernable impact upon minimizing climate changecaused by greenhouse gasses in our atmosphere, a method of manufactureis needed for economically making glass reflectors to be used in a solarconversion system according to the present invention. An objective ofthe present invention is to provide a method of manufacture that allowsthe use of low cost float glass for such glass reflectors. Anotherobjective of the present invention is to provide a method of manufacturethat is capable of high speed and fast production rates. The presentinvention includes an economical method of manufacture that is capableof producing glass reflectors made from float glass at low cost and at aspeed fast enough to accommodate the full output of a conventional floatglass factory production line in a continuous manufacturing process.

One feature of the present invention is that the power levels tominimize cost per watt are achievable in the intermediate range betweenabout 1 kW to about 20 kW, a range that has been barely touched byprevious solar concentration systems.

Concentration of solar radiation in accordance with the presentinvention is made by a rigidly co-aligned array of intermediate sizedish reflectors pointed substantially directly at the sun by a two-axistracker. Each intermediate size dish reflector powers a compactconverter at the focus of the reflector. An advantage of the presentinvention is a clean separation of compact energy conversion units froma plurality of relatively simple but inherently large energy collectionelements. In this way, aspects of mass production, transport assembly,upgrade and maintenance of the three major components, i.e., thereflector dishes, the trackers, and the conversion units, can beseparately optimized. In accordance with the present invention, opticalthroughput is maximized by using conversion units whose cross sectionalarea is much less that the area of the reflector dish needed to powerthem.

During the day, the sun rises in the east, moves across the sky, andsets in the west. There is a need to adjust the orientation ofreflectors so that they point in the direction of the sun as it movesacross the sky. In order to make a solar generation system that is costcompetitive, it is important to minimize the cost of the supportingmechanical structure used to track the sun. This is achieved inaccordance with the present invention by supporting multiple reflectorand converter units in a rigid, lightweight spaceframe. A spaceframe inaccordance with the present invention is a rigid, lightweight, opentruss structure that maximizes stiffness and strength while minimizingmass per unit area of collected light. The open truss structure extendsin three dimensions to achieve both high stiffness against bending bygravity and wind, and high strength to survive occasional very highwinds. Two dimensions extend across the area of sunlight collected, andthe third dimension extends in the perpendicular direction, along theoptical axis linking the reflector units below and the converter unitsabove.

In accordance with the present invention, the cost of the reflectors andtracker is further minimized by use of inexpensive materials, such asglass and steel. A feature of the invention is that individualreflectors take the form of a large glass monolith, as opposed to anarray of discrete abutting segments. Monolithic construction simplifiesglass reflector manufacturing and integration, and minimizes the numberof support points per unit area for a given glass thickness and mass perunit area (areal density).

One feature of the present invention is that the primary dish opticshave optical quality which results in only moderate degradation at theconcentration limit set by the diameter of the sun's disc, with thecapability of delivering solar radiation at high concentration,preferably 10,000 times, at the direct focus of a generally paraboloidalreflector of short focal ratio. Such high concentration is not alwaysrequired at the actual point of conversion, but along with accuratetracking of the rigid spaceframe structure, this level of concentrationprovides maximum flexibility for the design of conversion units toexploit the advantages of the solar concentration.

It is another feature of the present invention that the aspect ratio ofthe array of reflectors carried by each two-axis tracker is wide andlow. The horizontal extension provided in the present preferred designminimizes shadowing at low sun elevation by adjacent units in a field ofmultiple units, thus maximizing land use. This preferred design alsominimizes wind loading.

A feature of the present invention is that the drives have a highmechanical advantage, which advantageously minimizes parasitic loadsfrom drive motors. An additional feature of the present invention isthat the drive elements are applied at a large radius, whichadvantageously minimizes drive forces and mass of the drive components.

SUMMARY OF THE INVENTION

The present disclosure includes 1) an apparatus to deliver concentratedsunlight for production of electrical energy by conversion units, and 2)a method of manufacture for silvered glass reflectors suitable for usein making such an apparatus.

An apparatus in accordance with the present invention incorporates aplurality of large dish reflectors co-aligned in a rigid array, withprovision for holding a compact solar conversion unit above eachreflector dish. The supporting structure for the reflectors andconverter units is made as a 3-dimensional spaceframe, in which themembers linking the reflectors to the conversion units provide depth tothe structure and thus serve also to confer high overall stiffness. Thespaceframe together with the elevation bearings and drives provide anelevation structure for a two-axis, elevation-over-azimuth tracker withazimuth bearings and drives. When the tracker is positioned so that thereflector array is oriented to face the sun, strongly concentratedsunlight is delivered to the conversion units or converter units. Theinvention may be advantageously used with a variety of conversion units,including conversion units generating electricity through photovoltaicor thermal processes, or conversion units storing solar energy aschemical energy by thermal or photochemical reactions.

Preferably the converter units used with the apparatus are small, so asnot to significantly shadow the reflector dishes. An apparatus accordingto the present invention, including a set of converters and appropriateheat exchangers, forms a self-contained solar power conversion system.The present invention is especially adapted to provide solar energyconversion on a large scale by employing a plurality of suchself-contained solar power conversion systems, which may bemass-produced. Large scale arrays of solar power conversion systems maybe advantageously fielded or deployed in locations having strong directsunlight, such as the desert southwest of the United States.

Significant benefits and advantages are realized from using a pluralityof optimally sized reflector dishes on a single two-axis tracker inaccordance with the present invention. This configuration allowsefficient use of material (in terms of mass per unit power ofconcentrated sunlight). Preferably the array is balanced about theelevation bearing to minimize forces and moments due to wind andgravity. The reflectors are preferably arrayed in a rigid spaceframe,wherein the spaceframe has primary struts in all three dimensions, tomaximize overall stiffness, while at the same time providing nodesoptimally placed to hold: the individual reflector dishes; theconversion units; and the mounting points for elevation bearings anddrives. In the most preferred embodiments of the invention, thespaceframe consists largely of a rectangular framework of steel strutswith diagonal braces, thus providing very high stiffness to weightratio. Each large cell of the spaceframe houses a dish reflector belowand its associated conversion unit above.

The dish reflectors are preferably made as large monoliths ofback-silvered, low-iron float glass, each dish reflector being supportedby an open truss structure. Back-silvered glass reflectors are preferredfor this invention because of the superior protective and structuralfunctions of glass, and the very high reflectance of silver. Glass ispreferred also because of its rigidity and chemical and dimensionalstability, holding the true figure of the reflector on a wide range ofspatial scales. Monolithic construction of the glass is preferredbecause a large reflector is structurally more efficient when made as amonolith rather than as an array of smaller segments, and large glassmonoliths may be manufactured in very high volume at low cost, inaccordance with a method of manufacturing disclosed herein. Silver is apreferred reflecting material, but other materials such as aluminummight be substituted for silver with some degradation in performance.The longevity and stability of back-silvered glass reflectors providegood durability characteristics in solar applications exposed to outdoorweather conditions. Low-iron soda-lime silicate glass is a preferredmaterial because of its low solar energy absorbance. However, thoseskilled in the art will appreciate that other materials may besubstituted without departing from the spirit of the invention. Althoughthe word “glass” is used in the description below, unless otherwisestated, it should be understood that low-iron soda lime float glass ispreferred for this invention.

Preferably the dish reflectors have an axisymmetric concave figure and asubstantially square or hexagonal perimeter. The reflectors are arrayedin adjacent cells of the supporting spaceframe, matched in size andshape so as to minimize gaps and energy losses. The reflectors arepreferably both a relatively large optimum size and monolithic, tofacilitate both manufacture and cost at the minimum cost per unit area.An important feature of the design is that the monolithic reflectordishes are provided with a rolled edge, to increase rigidity and reducethe number of support points, and thus further reduce the cost ofmanufacture and installation of each glass monolith. A rolled edgeprovides structural advantages, and has not been previously exploited inthe shaping of solar glass reflecting dishes. The reflectors are madewith glass preferably of around 4 mm thickness, and supported on an opentruss by pads preferably spaced about every half meter. The reflector ispreferably bonded to the support pad with a compliant adhesive. Thedishes have a paraboloidal shape or another concave figure, optimizedaccording to the design of the generator or converter unit selected foruse near the focus.

To minimize the cost per unit area of the reflector and its assembly,the individual monolithic glass reflector dishes are optimized in sizeusing certain cost-benefit tradeoffs, and are preferably made as largeas practical consistent with other important considerations. The presentinvention advantageously provides significant cost savings by allowingglass reflector dishes to be made from a full standard width of floatglass production, generally around 3.3 m, which is strongly preferred.Each large square or hexagonal reflector is preferably attached to arigid, lightweight steel truss support. In one preferred example, thetruss provides on its upper surface a cradle of sixteen points in afour-by-four grid for attachment of a substantially square monolith ofapproximately 4 mm thickness, and the truss extends below to form fournodes in a square, to attach to lower corners of a unit cell of arectangular elevation spaceframe. For a preferred square reflectorformed from 3.3 m wide glass, the active area is approximately ninesquare meters, and thus the concentrated solar power delivered to eachdish focus is typically around 8 kW (for 1000 watts per square meterinsolation and an average reflectivity more than 90%). Conversion unitsmost appropriate for use with this preferred embodiment of the inventionare those which can operate efficiently at around 8.5 kW input powerlevel per unit.

Taking the above constraints together, a unit reflector cell for asubstantially square dished reflector preferably takes the form of aright prism with square cross section. A preferred embodiment for areflector is supported from the corners of the lower square end of theunit cell, and the compact converter or generator is supported from thecorners of the opposite, upper end by thin diagonal elements, preferablyin tension. The unit cell has diagonal members across all six faces, notjust the converter face, making it very stiff in all three deflectiondirections and all three twisting directions. A plurality of such unitreflector cells are linked in an array to form the deep, rigid elevationspaceframe. The elevation structure of the two-axis tracker preferablycomprises an n×m array of the cells configured to form a rigidspaceframe. This spaceframe may incorporate offsets between horizontalrows of units to improve mechanical performance. In either case, moststructural members act both locally to hold the unit converters in rigidalignment with their associated reflectors, and overall to provide avery stiff spaceframe. In order to achieve an optimum tradeoff betweenlow cost and performance, the cross sections of the truss struts arepreferably chosen such that the deflections under maximum operatingloads are nearing their budget for bending misalignment of the differentdish axes, while at the maximum survival wind load the stresses arenearing their failure limit. The configuration of the spaceframe andsupporting structure according to the present invention has beendesigned to achieve significant structural integrity at low cost, whichcan be a crucial consideration in a practical competitive system forgenerating solar electricity. The spaceframe structure in accordancewith the present invention yields the required performance with minimummass per unit power, and achieves the significant advantage of reducedoverall cost.

Two preferred embodiments of the apparatus are described in detailbelow, both employing an altitude over azimuth configuration. A first,and larger, described embodiment illustrates a configuration in whichthe unit cells are in a co-planar array, perpendicular to their commonoptical axis, and the azimuth bearing turns on a horizontal trackanchored to the ground. This first illustrated embodiment is moresuitable for larger scale units built upon a level site. The illustratedlarge embodiment of the invention has twenty-seven unit cells in aregular three-by-nine element spaceframe. The rigid elevation structureincludes a spaceframe with two braced C-rings underneath, forming avirtual pivot near its center of gravity. The load from the C-rings istaken almost directly to a circular azimuth track on the ground via asquat azimuth platform with four corner trucks. Each truck has an upwardfacing wheel(s) to support a C ring and a downward facing wheel(s)directly below that ride around the azimuth track.

In a second described embodiment of the apparatus employing an altitudeover azimuth configuration, the elevation structure comprises twostaggered rows of unit cells with the upper row set back and the lowerrow forward. The front bottom edge of the top row coincides with theback bottom edge of the top row to form a common structural memberextending the full length of the framework and passing through itscenter of gravity, thus providing a very stiff central member forsupport of the elevation bearing. An important feature of thisembodiment is that little structure is required in addition to the twolines of cells to form a complete and very stiff spaceframe andelevation structure. In this illustrated embodiment, each row ofreflector cells includes also a central narrow cell to accommodate theelevation axis bearing. Azimuthal motion is about a pedestal centeredbelow the elevation axis. This pedestal pivots on a post that preferablytakes the form of a steel truss set in a hole in the ground with crushedrock or gleaned land rubble, preferably avoiding any use of concrete inthe entire assembly in order to minimize costs.

Silvered glass reflectors for incorporation into a solar concentratorfor use in the above-described embodiments, and which have utility inother alternative embodiments, may be manufactured according to low costmethod using float glass. This method of manufacture contributessignificantly to reduce the overall cost of a solar power system.

In accordance with the present invention, the starting point for thereflector manufacturing process is float glass. Float glass is stronglypreferred because of its high quality and low cost. Float glass alsoemploys mature technology that can allow production of glass reflectorsin a very large volume. In order to have a significant impact on thereduction of greenhouse gasses and carbon pollution in the atmosphere,it is desirable to have a method of manufacturing glass reflectors forsolar energy conversion systems at a sufficiently low cost and at asufficiently high volume to facilitate the mass production of systemsfor generation of very high power levels of electricity. The processhere described may be advantageously integrated into a float glassproduction line, so that freshly formed hot glass from the float glassproduction line can be shaped before it is annealed and cooled. Themanufacturing process according to the present invention provides thecapability of utilizing the entire output of a float glass factory toproduce glass solar reflectors made in one continuous process orproduction line. The manufacturing process is thus designed to work atvery high speed, for example allowing a new glass reflector to be madeevery ten seconds, and is capable of accommodating the typicalproduction rate from a float glass factory for 3.3 m square sheets 4 mmthick. The manufacturing process may alternatively be conducted off-lineat a slower rate if desired, but the capability of high production ratesin accordance with the disclosed method of manufacture is a significantadvantage provided by the present invention.

The first step in the manufacturing process comprises cutting a freshlyformed and still hot but rigid float glass ribbon into reflector sizedpieces. This is followed by the step of positioning each reflector-sizedpiece of glass above a full body mold. Each reflector-sized piece ofglass is then heated while its corners are restrained against lateralmotion. As the reflector sized piece of glass softens in the heatedenvironment, the glass sags and stretches, touching and conforming tothe perimeter of the mold and otherwise freely descending to take up theapproximate shape of the dished reflector, while keeping its smooth,specular finish. The corner restraints are provided to preventwrinkling, by forcing the glass to stretch everywhere as it sags. Thedesired shape of the dish is taken on by the reflector-sized piece ofglass when it reaches and lightly touches across the full body of themold. This method achieves the goal of precise shaping of thereflector-sized piece of glass without significantly degrading theoptical smoothness of either the front or back surface of the glass. Astainless steel mold is preferably used with cusped grooves machinedinto it, so as to accurately define the figure while providing theadvantage of keeping the fraction of the glass area marred by contactwith the mold to well under 1%. While the reflector sized piece of glassis still soft and the concave dish is being shaped, forming tools areused at the edges of the glass to press back the glass to form rollededges against a rounded edge of the mold. As soon as the shaping iscomplete, the reflector-sized piece of glass may be rapidly cooled tostiffen it to the point where it can be removed from the mold withoutsignificant distortion. In a continuous production line, the mold may berouted back to the beginning of the production line and recycled toshape the next reflector. It is desirable to minimize thermal cycling ofthe mold, because thermal cycling could shorten the lifetime of themold. The disclosed method of manufacturing achieves the desirableresult of minimizing thermal cycling of the mold by use of cuspedgrooves in the operative surface of the mold corresponding to the pieceof glass that is being molded, and the grooves minimize thermalconduction from glass to mold. In accordance with a preferred method ofmanufacture, thermal cycling of the mold is further reduced by coatingthe operative mold surface proximate to the piece of glass that is beingmolded so as to be highly reflective to heat radiation fluxesexperienced during heating and cooling.

Radiative heat transfer is a preferred method to heat and cool a pieceof float glass that is being molded, in order to achieve desirable fastrates of production. Preferably many molds will be in usesimultaneously, configured for example in a production line, to keep upwith a typical factory rate of sheet production of float glass. The moldcycle according to a preferred manufacturing process comprises:positioning a reflector size glass sheet into a correspondingrelationship with a mold; heating the glass sheet by rapid radiativeheating to soften the glass sheet; maintaining the glass sheet in aheated environment until the glass sheet is shaped into a reflector bysagging, stretching and settling the glass sheet across the mold;shaping the edges of the glass sheet; cooling the glass sheet by rapidradiative cooling; removing the shaped glass reflector for annealing andcooling; and returning the mold for reloading. The disclosedmanufacturing process is capable of rapid production of glassreflectors, and is adapted to achieve an estimated processing time ofabout three minutes for completion of these steps. The advantages ofproduction rate offered by the disclosed manufacturing process becomemore apparent by considering that a new glass sheet may be processedinto a shaped glass reflector about every ten seconds, by using twentyor more molds in a production line in accordance with the presentinvention. In such a production line using twenty molds, a productionline length of approximately 100 meters provides several minutes ofthermal processing before a mold is returned, based on a production linelength that is about twenty or more times the mold-to-mold separation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an array of reflectors mounted on amoveable support with a two-axis tracking system, and having distributedsolar converter units.

FIG. 2 shows a side view of an embodiment according to FIG. 1, viewedalong the elevation axis of the two-axis tracking system.

FIG. 3 shows a top view of an embodiment according to FIG. 1 viewedalong the optic axis, as seen from the sun direction.

FIG. 4 is an enlarged perspective view (partial) of an embodimentaccording to FIG. 1, showing a paraboloidal reflector and an especiallypreferred converter unit having a ball lens.

FIG. 5 is a perspective view of an especially preferred glass ball lensfor use at the focal point of a solar converter unit in accordance withthe present invention, and showing an especially preferred array ofsecondary concentrating reflectors appropriately positioned relative tothe glass ball lens, with all of the supporting structure removed forthe sake of clarity.

FIG. 6 is a plan view of a plurality of arrays of reflectors accordingto the preferred embodiment shown in FIG. 1, in a configuration suitablefor a solar farm.

FIG. 7 is a perspective view of an alternative embodiment mounted on amoveable support with a two-axis tracking system.

FIG. 8 shows a side view of an embodiment according to FIG. 7 with atwo-axis tracking system set at low elevation angle, viewed along theelevation axis.

FIG. 9 shows a side view of an embodiment according to FIG. 7 with atwo-axis tracking system set at 45° elevation angle, viewed along theelevation axis.

FIG. 10 shows a side view of an embodiment according to FIG. 7 with atwo-axis tracking system set at high elevation angle, viewed along theelevation axis.

FIG. 11 shows an embodiment according to FIG. 7 when pointing at thehorizon viewed along the optic axis, showing details of an azimuthbearing and pedestal foundation.

FIG. 12 shows an alternative embodiment pointing at the horizon viewedalong the optic axis, showing details of an azimuth bearing and pedestalfoundation.

FIG. 13 is a perspective view of an embodiment according to FIG. 7showing details of elevation and azimuth drives.

FIG. 14 is a perspective view of an embodiment according to FIG. 7showing details of an azimuth chain drive.

FIG. 15 is a perspective view showing details of an elevation bearingfor an embodiment according to FIG. 7.

FIG. 16 is a plan view of a plurality of arrays of reflectors accordingto an embodiment shown in FIG. 12, in a configuration suitable for asolar farm.

FIG. 17 is a perspective view of a plurality of arrays of reflectorsaccording to an embodiment shown in FIG. 12, in a configuration suitablefor a solar farm.

FIG. 18 is a view from the sun direction of a plurality of arrays ofreflectors according to an embodiment shown in FIG. 12, in aconfiguration suitable for a solar farm.

FIG. 19 is a perspective view of an embodiment of a single solarconversion unit, showing details of a structure to support a converterunit above a square reflector.

FIG. 20 is a perspective view of an alternative embodiment of a singlesolar conversion unit, showing details of an alternative structuresupporting a converter unit above a hexagonal reflector.

FIG. 21 is a perspective view of a preferred embodiment of a reflectorunit for use in a solar conversion unit according to one embodiment ofthe invention, showing a glass reflector and its supporting structure.

FIG. 22 is a perspective view of supporting structure for a monolithicglass reflector according to the embodiment illustrated in FIG. 21,showing the supporting struts and nodes with the glass reflectorremoved.

FIG. 23 is a plan view of supporting structure for a monolithic glassreflector according to the embodiment illustrated in FIG. 21, showingdetails of the cradle of support points and the uppermost supportingstruts and nodes shown in FIG. 22.

FIG. 24 is a perspective view of a contour map for a calculateddeflection under an axial one g gravity load for 4 mm thickness, showingthe change in slope angle across the surface of a glass reflector causedby the loading slope deviation.

FIG. 25 is a cross sectional view of a piece of flat glass to be shapedinto a reflector for use in the embodiments illustrated in FIG. 1 andFIG. 7.

FIG. 26 is a cross sectional view of the piece of glass according toFIG. 25, placed on a mold.

FIG. 27 is a cross sectional view of a piece of glass after the stepshown in FIG. 26, where the piece of glass has been softened byradiatively transferred heat and partly shaped to the mold.

FIG. 28 is a cross sectional view of a piece of glass after the stepshown in FIG. 27, where the piece of glass has been fully conformed tothe mold shape.

FIG. 29 is a cross sectional view of a molded piece of glass after thestep shown in FIG. 28, where the piece of glass is being cooled byradiative transfer.

FIG. 30 is a cross sectional view of the molded piece of glass accordingto FIG. 29, where the molded piece of glass has been removed from themold.

FIG. 31 is a perspective view showing a square mold with a flat sheet ofglass held above the mold.

FIG. 32 is a perspective view showing a square mold with a partiallycurved sheet of glass suspended above the mold.

FIG. 33 is a perspective view showing a square mold with a fully formedsheet of glass resting on the mold, with corner supports still in place.

FIG. 34 is a perspective view showing a square mold with a fully formedsheet of glass resting on the mold and corner supports removed.

FIG. 35 is a partially cutaway plan view of a square mold showing theunderlying supporting framework and corner supports.

FIG. 36 is a partially cutaway perspective view of a square mold showingcorner supports and some front panels attached.

FIG. 37 is a cross sectional detail view of a corner support holding aflat sheet of glass prior to shaping.

FIG. 38 is a cross sectional detail view of a corner support holding apartially formed sheet of glass.

FIG. 39 is a cross sectional detail view of a corner support holding adished sheet of glass resting on the mold with the corner supports stillin place.

FIG. 40 is a cross sectional detail view of a dished sheet of glassresting on the mold with the corner supports removed.

FIG. 41 is a partially cutaway perspective view detailing a section atthe edge of a mold, with a forming tool positioned above the mold.

FIG. 42 is a partially cutaway perspective view detailing a section atthe edge of a mold with partially shaped glass in place.

FIG. 43 is a partially cutaway perspective view detailing a section atthe edge of a mold with glass in place and formed into a rolled edge bya shaping tool.

FIG. 44 is a partially cutaway perspective view detailing a section ofglass curved and shaped with a rolled edge by the molding processaccording to the present invention.

FIG. 45 is a schematic diagram showing a cutaway view of a section of amold surface with grooves and cusps.

FIG. 46 is a schematic diagram showing a cutaway view of a mold sectionwith shaped glass touching the cusps.

FIG. 47 is a cross sectional view of a cusped mold detailing shapedglass resting on the cusps after completion of shaping.

FIG. 48 is a schematic diagram illustrating a plurality of molds forshaping glass reflectors in a production line adapted to manufactureglass reflectors at a rate corresponding to the rate of output of afloat glass production line.

FIG. 49 is a schematic diagram of an electrical control system for atwo-axis solar tracker.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1, FIG. 2 and FIG. 3 illustrate a presently preferred embodiment ofthe invention comprising a plurality of unit reflector cells 34 withlarge paraboloidal reflectors 1 held co-axial in a rigid framework 100.In the illustrated example, an array of twenty-seven unit reflectorcells 34 is shown in a rectangular grid that is three rows by ninecolumns, but other configurations and arrangements may be used with aplurality of concentrators arranged in various other configurations. Itis desirable to have the capability of pointing the array of reflectors1 at the sun as the sun moves across the sky during the day. In order toaccomplish this, the common axis of the reflectors 1 is oriented to thesun by a two-axis tracker 107. In this exemplar embodiment, a flat truck101 turns on a circular track 102 on the ground to provide azimuthalrotation of the framework 100 supporting the paraboloidal reflectors 1.Elevation motion is provided by two C-rings 103 attached to the rigidframework 100, which turn about a virtual axis 110 on upper wheels 104above and adjacent to truck wheels 105. Diagonal supports for theelevation ring 109 and diagonals in the structure 48 provide stiffness.In this preferred embodiment, the paraboloidal reflectors 1 are nearlysquare or substantially square with clipped corners 106, in order tominimize the reflective area that would otherwise be lost, and arearrayed in a rigid framework 100 of square or rectangular symmetry, sothat nearly all of the sunlight intercepted by the entire array of unitreflector cells 34 is delivered to the respective paraboloidal focus 28of each of the reflectors 1.

FIG. 4 and FIG. 5 are perspective views of a unit reflector cell 34 witha converter in the form of a photovoltaic generator 4 and paraboloidalreflector 1. Sunlight entering the system enters as incoming solarradiation or rays 2 which are reflected by the mirror 1. The mirror 1 isshaped so that reflected solar radiation or rays 3 are directed towardthe focus 28, where a photovoltaic generator 4 is located, in thisillustration including a ball lens 5. The minor 1 preferably has asubstantially paraboloidal reflecting surface that reflects as much ofthe sunlight 2 as possible and concentrates it at the location of thegenerator 4. The mirror 1 is preferably fashioned with a thin silvercoating on the backside of low iron glass. However, a front-coatedmirror, while not preferred, may be provided as an alternative, andalternative reflective coatings may be used on the glass with othermetallic or dielectric coatings. In addition, the minor may befabricated from alternative materials that nevertheless provide thedesired reflection of sunlight toward the generator 4. Although thereflector 1 is preferably substantially paraboloidal in configuration,other curved surfaces may be employed as well with some degradation inperformance. A paraboloidal shaped reflector 1 is preferred, because itwill reflect parallel incoming rays 2 to a focal point 28. Since theincoming solar radiation is not precisely in the form of parallel rays2, alternative shapes of the reflective surface 1 may be utilized toreflect rays 3 to a focus 28 at which the generator 4 is located.However, the combination of elements comprising the mirror 1 of thepreferred embodiment described herein is believed to provide an optimumtradeoff between cost and performance. In a preferred embodiment whereinthe reflector 1 is substantially paraboloidal, deviation of the shape ofthe reflective surface from the desired paraboloidal shape may becharacterized as surface slope errors in the reflective surface of theprimary paraboloidal mirror 1, and any converter to take full advantageof the present invention should provide some tolerance for such errorsas well as some tolerance to tracker pointing errors. This is adesirable feature in achieving a degree of cost effectiveness inmanufacture and operation.

An example of a converter, in the form of a photovoltaic generator 4,for use at the focus 28 of the paraboloidal minor 1 is shown in theperspective view of FIG. 5. The generator 4 comprises a ball field lens5 and has a plurality of secondary reflectors 45 concentrically locatedabout the ball field lens 5, wherein the plurality of secondaryreflectors 45 direct the concentrated light onto a plurality ofconcentrator photovoltaic cells 15 as shown in FIG. 5. The photovoltaiccells 15 generate electricity directly from solar radiation. Thestructure, function and operation of the generator of FIG. 5 and otherphotovoltaic generators of this type is described in more detail inapplication Ser. No. 12/463,016, entitled “Photovoltaic Generator with aSpherical Imaging Lens for Use with a Paraboloidal Solar Reflector,” byRoger P. Angel, the entire disclosure of which is incorporated herein byreference.

The concentrated sunlight 3 that passes through the ball lens 5 forms animage of the primary paraboloidal reflector 1 on a concave sphericalshaped surface 9 where secondary reflectors 45 are located in a positionconcentric with the spherical lens 5. FIG. 5 shows only the ball lens 5,bypass diodes 30, and a multiplicity of square photovoltaic cells 15placed immediately behind secondary reflectors in the form of mirroredframes 45 with approximately square inputs. All other structure isomitted for the sake of clarity. The plurality of photovoltaic cells 15are electrically connected together in series to combine the electricalpower produced by each cell 15 into a total output of electricity fromthe generator 4. The generator 4 includes a system (not shown) toactively cool the cells 15 by water pumped to a radiator, for convectivecooling to the air.

A significant aspect of the generator 4 is that the intensity of thesolar radiation 3 across all the photovoltaic cells 15 is stabilized bythe ball lens 5 against pointing errors of the tracker 107, for a designthat is tolerant of surface slope errors in the reflective surface ofthe primary paraboloidal minor 1. Even if the tracker 107 is not pointeddirectly at the sun, the light intensity of the image formed by the balllens 5 at the entrance to the prismatic reflectors 45 remains relativelyconstant and provides equal illumination of all of the cells 15 for asignificant range of pointing errors. Another significant aspect of thegenerator 4 is the action of the secondary reflectors 45 in furtherimproving tolerance to mispointing and in opening substantial gapsbetween adjacent cells 15. The gaps provided between adjacent cells 15allows placement of bypass diodes 30 and interconnecting wiring in closeproximity to the cells 15, without interference to either their opticalinput or to their electrical and thermal outputs. Thus the generator 4of FIG. 5 provides for the high solar power input from the concentratorof this invention, both by flux stabilization, so as to ensure highpower output from many cells 15 connected in series, and by a clearthermal path to remove the waste heat from a close array of intenselyilluminated cells 15.

FIG. 2 illustrates the elevation support and drive of the rigidframework 100 according to FIG. 1, in a view seen along the direction ofthe elevation axis 110 perpendicular to the plane of the drawing, andpassing through the center of the arc of the C-ring 103. Rotation aboutthis elevation axis 110 is defined by the motion of the C-rings 103 ontheir supporting wheels 104, not by any physical bearing on the axis110. The C-rings 103 are braced by in-plane struts 109 that link tonodes 11 of the unit reflector cells 34 in the rigid framework 100. FIG.3 illustrates the rigid framework 100 according to FIG. 1 as seen alongthe direction of the optic axis, and shows how the substantially squaredish reflectors 1 pack efficiently in the rigid framework or spaceframe100 to capture most of the sunlight 2 incident on the elevationspaceframe 100.

FIG. 4 illustrates a preferred embodiment of a thermosiphon for passiveremoval of waste heat from the generator 4. Heat at the generator 4boils a working fluid, causing vapor to rise up the thermosiphon tube 39to a convective structure 24 with air-cooling located in the upperquadrant of the diagonal support braces 38. In the illustrated preferredembodiment, the convective structure 24 remains in an orientation abovethe generator 4 over the entire range of elevation angles contemplatedin practice for the elevation axis 110, so that the condensed fluid willreturn to the generator 4 by gravity. This provides a cost effectivedesign for the circulation of cooling fluid in the thermosiphon tube 39and the convective structure 24 without adding the additional cost andmaintenance of a pump.

FIG. 6 illustrates quantitatively a preferred placement of multipletracker units 107 of this embodiment, laid out in a grid of equilateraltriangles, sized to allow clearance for full azimuth rotation with nopossibility of collision. A preferred ratio of three for projectedhorizontal length to projected height is advantageous, as shown in thisillustrated example, because this ratio minimizes self shadowing at lowsun elevation in the early morning and late afternoon, and alsominimizes wind loading. Given rows oriented in the north-southdirection, when the sunlight is coming from due east or due west, thereis then no self shadowing loss for solar elevations above tan⁻¹(⅓)=18.4° degrees, and only ⅓ of the unit cells would be shadowed for12.3° solar elevation. The ratio of reflector area to land area for theplacement illustrated is approximately 25%. Because in this preferredembodiment all reflectors 1 are coplanar and in an open truss structure100, wind loading may be minimized by stowing the tracker 107 in thezenith pointing position with the reflector plane horizontal in veryhigh winds. Maintenance roads 112 are shown placed every two rows in onedirection. By suitable azimuth and elevation rotation, ready access forfront or back service is thus provided for all trackers 107.

A second presently preferred embodiment of the invention is shown inFIG. 7. This alternative embodiment comprises a plurality of largeparaboloidal reflectors 1 held co-axial in a rigid framework 50, butwith a smaller array of eight unit cells 34 in a configuration of tworows 51 and 52 and four columns. In order to point the array ofreflectors 1 at the sun, the common axis of the reflectors 1 is orientedto the sun by a two-axis tracker with altitude over azimuth bearings. Asshown in FIG. 11, azimuthal motion is provided by a vertical axisbearing 78 and 79 set on a pedestal mount 67. The reflector array isturned in elevation by a horizontal-axis elevation bearing 58 set abovethe azimuth bearing 78 and 79.

In the alternative embodiment shown in FIG. 7, the reflectors 1 arepreferably generally square or substantially square with clippedcorners, and each reflector 1 is housed in a unit reflector cell 34,taking the form of a square-based, right prism with provision to supporta unit generator 4 near the focus 28. As shown more clearly in FIG. 11,(which is a view along the optical axis), the elevation spaceframe 50 isconfigured so that the outline as seen from the sun is rectangular andnearly all of the sunlight intercepted by the entire array is directedto the focus 28 of the plurality of the reflectors 1.

The forward/backward displacement of the two staggered rows 51 and 52 isillustrated most clearly in the views along the elevation axis shown inFIG. 8, FIG. 9 and FIG. 10. In FIG. 8, the elevation angle is low, inFIG. 9 the elevation angle is at 45°, and in FIG. 10 the elevation angleis high. The upper row of cells 51 is set forward and the lower row 52is positioned back so that the front lower edge of the upper row 51coincides with the back upper edge of the lower row 52, and a commonstructural member 53 is provided in the framework 50. This commonstructural member 53 is made coincident with the elevation axis, andpasses close to the center of gravity of the complete elevationstructure as detailed in FIG. 7. The elevation spaceframe 50 iscompleted with the addition of front and rear stiffening members 56 and57, respectively, between the upper row 52 of cells and the lower row51. The front stiffening members 56 link upper front corners 14 of thetwo rows 51 and 52 of cells, and the rear stiffening members 57 linklower back corners 11.

Referring to FIG. 7, by staggering the two rows 51 and 52 of cells 34,the spaceframe 50 is naturally balanced about the elevation axis andthus efficiently supported by a central pedestal 67. Moreover, the powerneeded to move or rotate the rigid framework 50 in elevation isminimized. A second advantage of the staggered rows 51 and 52 of cells34 is that the cross-section perpendicular to the rows 51 and 52 of theresulting spaceframe 50, which includes the perimeter stiffening struts56 and 57, is maximized in two dimensions. This results in highstiffness against bending about the central bearing 58, no matter whatthe elevation angle may be. While the structure in this exemplarembodiment has four reflector units 34 in the upper row 52 and fourreflector units 34 in the lower row 51, it will be also apparent tothose skilled in the art after having the benefit of this disclosurethat staggered rows of different lengths, for example two or six, mightprovide good results in practice, depending on the survival windstrength desired and appropriate for a given site, and depending onprevailing manufacturing costs. For example, FIG. 12 illustrates analternative embodiment similar to that shown in FIG. 7 and FIG. 11, butwith six reflector units 34 in each of two staggered rows 51 and 52.

Returning to the embodiment shown in FIG. 7 and FIG. 11, the elevationaxis bearing 58 in this embodiment connects to the elevation spaceframe50 via two nodes on the central structural axis, the details of whichcan be better described with reference to FIG. 11. Space for the bearing58 is created by inclusion of a narrow central cell 59 in the lower row51 of four unit reflector cells 34, and by inclusion of a narrow centralcell 60 in the upper row 52. FIG. 15 shows how high structuralefficiency is achieved because stiff elevation nodes 61 and 62 thatsupport the ends of the elevation bearing 58 are formed as an inherentpart of the rigid spaceframe 50 that supports and links the reflectors1. For clarity, FIG. 11 shows only a few of the struts supporting theelevation bearing 58. FIG. 15 shows in detail how the two ends of theelevation bearing 58 are supported at the two very stiff nodes 61 and 62formed at the intersection of corner struts of the four inner unitreflector cells 34. No additional steel members are needed to stiffenthis support 61 and 62, or to place the elevation bearing 58 at thecenter of gravity. Linking members 56 and 57, shown for example in FIG.9, stiffen the two lines of cells to complete the full elevationstructure that turns about the elevation bearing 58 on the centralpedestal 67.

An elevation drive arc 63 is shown in FIG. 13. The elevation drive arc63 is securely attached on one side of the narrow center cells 59 and 60(shown in FIG. 7) from the two lower nodes 64 and 65 (shown in FIG. 9),providing for motion over a range of 90° from zenith to horizon pointingas shown by comparing FIG. 8, FIG. 9 and FIG. 10. In the illustratedembodiments, the drive arc 63 has a radius equal to the width and depthof the unit cells 34, which are the same in the illustrated embodiment.

The azimuth bearing for the embodiment shown in FIG. 7 and in FIG. 11takes the form of a central pedestal 67 that turns about a fixedvertical shaft 66. The axial load is taken by a thrust bearing 78positioned in the upper portion of the central pedestal 67, or at thetop of the shaft 66. Lateral loads are preferably taken by two plasticjournal bearings 78 and 79, one at the top and one at the bottom of theshaft 66. The azimuth drive acts through a large 360° drive wheel 68attached to the lower end of the central pedestal 67. In the illustratedexample, the drive wheel 68 is shown with the same large radius as theelevation drive. However, the present invention is not so limited, andthose skilled in the art will appreciate that other radii for the drivewheel 68 can be used.

Referring to FIG. 13, a preferred drive for azimuth motions in apresently preferred embodiment is by an electrically driven chain 70,which is shown in more detail in FIG. 14. In order to rotate thespaceframe 50 around the azimuth shaft 66, a drive force acts through anazimuth drive chain 70 stretched around and securely attached to theazimuth wheel 68. A rider sprocket wheel 74 may be used to adjust ormaintain a desired tension in the drive chain 70. In the illustratedexample shown in FIG. 14, the azimuth wheel 68 is supported by spokes76. The elevation drive arc 63 (shown in FIG. 13) and the azimuth drivewheel 68 are preferably constructed simply and at low cost from curvedstructural channels 77 as illustrated in FIG. 14.

A preferred drive for elevation motions in the illustrated embodimentsis by an electrically driven chain 69, as shown in FIG. 13. The drivechain 69 used for the elevation drive may be similar to the drive chain70 used for the azimuth drive shown in FIG. 14. In the elevation drive,a drive force acts through an elevation drive chain 69 attached to theends of the elevation arc 63, in order to rotate the spaceframe 50around the elevation bearing 58. The elevation chain 69 is attachedsecurely to the ends of the arc 63. The drive force is applied to theelevation drive chain 69 by a sprocket wheel turned through an electricmotor and gear box, similar to the sprocket wheel 71 and electric motorand gear box 73 shown in FIG. 14 for the azimuth drive. The elevationarc 63 is supported by a small truss 75.

As shown in FIG. 14, the drive moment is applied to the azimuth drivechain 70 by a sprocket wheel 71 turned through an electric motor andgear box 73. In order to minimize the tension in the chain 70 andincrease the overall gear ratio, the radius of the azimuth drive wheel68 is made large. At least one idler sprocket 74 is needed in the chaindrive, so that the engagement of the chain 70 on the drive sprocket 71is greater than 120 degrees. In this way the wear on the chain 70, andthe chance of the chain 70 slipping over the drive sprocket 71 teeth athigh torque, are both reduced. In a solar energy application, the rateat which the azimuth drive and the elevation drive must be moved inorder to track the movement of the sun is relatively slow, and the drivemechanism can take advantage of high mechanical ratio reductions, andallow the use of lower cost drive motors 73. Preferred mechanical ratioreductions of approximately 100 to 1 can be realized in practice, almostan order of magnitude larger than for a traditional chain drive withsprockets. Preferably the idler 74 or end of the chain 70 will betensioned by a spring take up mechanism. Although not shown in detail,those skilled in the art will appreciate that the elevation drive chain69 is driven by a similar arrangement, and the radius of the elevationdrive arc 63 is made large for the same reasons that the azimuth drivewheel 68 is provided with a large radius. The elevation drive chain 69is preferably tensioned by a spring take up mechanism attached to theend of the chain 69.

Taken together, the use of large arc radii for the azimuth drive wheel68 and the elevation drive arc 63 to reduce the required drive force,together with a chain drive with engagement of many sprocket teeth 71,will result in reduced cost, complexity and size of both the azimuthdrive elements and the elevation drive elements.

During operation it is desirable to track the movement of the sun fromeast to west across the sky so that the rigid framework 100 is pointedin the direction of the arriving solar radiation 2. Referring to theschematic diagram of FIG. 49, precise solar pointing may be achieved ifthe tracker 107 is servoed to elevation and azimuth error signalsderived from an optical sun tracking sensor 121, rigidly attached to theframework 100. A preferred form of sun tracking sensor 121 comprises asilicon quad cell illuminated by sunlight passing through a pinholeabove the silicon quad cell. Preferably the orientation is sensed alsoby a gravitational and magnetic field sensor 122 that does not requiresunlight. FIG. 49 illustrates schematically signals from sensors 121 and122 connected to a computer 120. Drive control signals from the computer120 are converted into drive power by an interface 125, and the electricpower is carried by an azimuth cable 124 to the azimuth drive motor 73and by an elevation cable 123 to the elevation drive motor 73.

Preferably the orientation of the framework 100 toward the sun ismaintained under cloudy conditions by reference to a sensor thatprovides absolute pointing data from a transducer 122 that reads gravityand magnetic fields, thus avoiding errors from settling of the pedestalfoundation 40, stretching of the chains 69 or 70, or bending from astrong steady wind. Alternatively, conventional telescope drivealgorithms may be employed to compute the position of the sun, basedupon the GPS coordinates of each individual tracker 107, and utilizingaccurate time and date information provided to the computer 120.Accuracy can be improved by synching the computed position of the sun,if necessary, with the position indicated by sun tracking sensor 121when sunlight is available, and such synchronization can effectivelyaccount for errors from settling of the pedestal foundation 40,stretching of the chains 69 or 70, or bending from a strong steady wind.Conventional computer controlled drive systems similar to conventionaltelescope GOTO systems may be used to point the rigid framework 100 inthe correct direction and to track the movement of the sun, includingencoders providing azimuth angle information and elevation angleinformation to the computer 120. It should be understood that theillustration shown in FIG. 49 is schematic only, and that the driveelectronics may be attached to the framework 100 or elsewhere.Preferably the electric power for the azimuth and elevation drive motors73 and active cooling systems (if any) will be furnished by anuninterruptible power source. A suitable source may be obtained byproviding photovoltaic converters over the sun-facing side of the narrowcenter cell 60 of the upper row, which may be used in conjunction withstorage batteries.

A preferred pedestal to support the azimuth bearing for this embodimentis shown in cross section in FIG. 11 and FIG. 12. The pedestal takes theform of a welded triangular steel truss 40 that extends into a hole 41,which may be drilled in the ground 42, for example, with a conventionaltruck mounted auger. Alternatively, the hole may be excavated by anumber of conventional means known to those skilled in the art. Afterinsertion of the truss or post 40, the hole 41 is preferably backfilledusing inexpensive dry materials 43 such as crushed rock or overburdenfrom mining, without use of cement binder. The hole and truss dimensionsare sized so that the dry materials will take the relative concentratedload from the truss 40 and transfer it to a much larger area of soil 42,to ensure survival in high wind. Unbonded rock is preferred instead ofconcrete because of its much lower cost and carbon footprint. Of course,those skilled in the art will appreciate that concrete can be used ifthe typically higher cost of such material can be tolerated in aparticular application. Some small settling motion of the pedestal 40 isacceptable, and low accuracy in the initial placement of the pedestal 40may be tolerated, since the tracker drive 111 will use solar tracker 121and absolute sensors such as accelerometers and/or frequentlyrecalibrated or synchronized sensors to position the rigid framework100. Alternatively, those skilled in the art will appreciate that othertruss foundation designs may be used without departing from the spiritand scope of the present invention.

FIG. 16, FIG. 17, and FIG. 18 show a preferred placement for theembodiment of the dish array 111 that is illustrated in FIG. 12. FIG. 16shows in plan view a regular triangular grid allowing clearance for fullazimuth rotation with no possibility of collision, similar to the layoutillustrated in FIG. 6. The relatively small, post mounted units requirevery little site grading or preparation. They are advantageouslysuitable for installation on sloping land as well as flat land, or inirregular parcels.

Preferred Dimensions

An important consideration in the design of a preferred embodiment isthe size and manufacture of the primary paraboloidal reflectors 1,especially since cost is an important factor. The present inventionincludes an optimization of the size of the reflectors 1. For reflectors1 made from structural, back-silvered glass, a size of about threemeters (3 m) is preferred as a generally optimum size. This size ispreferred because it is the largest shippable size that can be made froma single monolith of flat glass manufactured by a conventional floatprocess. Monolithic reflectors 1 at this size may provide the lowestcost per unit area for tracking reflector systems in accordance with thepresent invention.

In operation, preferably multiple dish-generator units 34 ofapproximately 3 m size are mounted on each tracker 107, to obtain thelowest total cost per unit area. Each square reflector 1 has areflecting area of about 9 m². For a nominal solar flux of 1000 W/m² andreflectivity of 94%, the nominal solar power delivered to each converterunit 4 is about 8.5 kW.

Supporting structures 100 of steel for the preferred embodiments ofoptimal two-axis trackers 107, having 3 m glass reflectors 1 of 4 mmthick glass, should provide satisfactory performance in practice. For atwenty-seven reflector 1 embodiment, the mass of steel for the fullsystem, including the fixed track, is 34 kg per square meter ofcollecting area, and the deflection a maximum of 0.1° when zenithpointing. For the eight reflector 1 embodiment, the mass of steel forthe full mount 107, including the fixed pedestal, is 25 kg per squaremeter of collecting area, and the maximum deflection when zenithpointing is 0.06°.

Details of a Preferred Structural Design for a Unit Reflector/ConverterCell

The preferred embodiments of the invention use, as building blocks, aplurality of unit reflector cells 34. The unit reflector cells 34comprise a reflecting dish 1 to direct energy to the focal region 28 anda mechanical structure to support the dish 1 and to hold the converterunit 4 in rigid alignment near the focal region 28. A major element ofthe mechanical structure is a truss in the form of a right prism whosecross section and end faces are preferably square or hexagonal, asillustrated in FIG. 19 and FIG. 20. FIG. 19 shows a unit cell 34 withsquare cross section, as adopted in a preferred embodiment describedabove, and FIG. 20 shows an alternative unit cell 34 having a hexagonalcross section. A substantially square (FIG. 19) or hexagonal (FIG. 20)dished mirror 1 is supported from the corners 11 of the lower prism faceby a dish support structure, described below. The dish axis 7 passesthrough the dish center or vertex 6. A compact converter 4 that obscuresonly a small fraction of the primary collector is located above,centered on the dish axis 7, and supported from the upper corners 14 ofthe upper prism face formed by members 13. The upper face is of the samesize as that formed by members 10, and is parallel to it. Both the upperand lower face centers lie on the dish axis 7. The converter 4 iscentered on the dish axis 7, and is preferably supported at the lowerend of a central axial converter support member 18. The convertersupport member 18 is preferably supported by diagonal tension elements17 from the upper corners 14. Four (FIG. 19) or six (FIG. 20)perpendicular elements 16 link the upper and lower squares. Preferablythe side faces of the prism are braced by diagonal elements 48. Theglass support truss 12 closes the bottom side of the structure as wellas supporting the primary paraboloidal reflector 1.

The concentric placement of the converter support 18 is appropriate forembodiments in which waste heat is removed by actively pumped coolingliquid, rather than passively by a thermosiphon 24. In an active coolingsystem, the cooling liquid is circulated through a heat exchanger whichmay be located directly above the generator 4 in FIG. 19 or FIG. 20, orat another location that does not obscure the incoming sunlight, reachedby pipes along the diagonal supports 17 and 18.

The most preferred embodiment of the reflector dish 1 is a single,slumped, back-silvered reflector of float glass attached to an opensteel truss. A cradle of well spaced upper nodes of the truss holds theshape of the glass reflector 1 on large spatial scales against gravityand wind forces, through bonded pads. The lower nodes transfer the loadto the lower corners 11 of a unit cell 34 of the main elevationframework 100.

The glass support truss 12 linking the silvered glass reflectors 1 tothe corner supports 11 is optimized in accordance with the size, shapeand thickness of the glass reflector 1 in any given embodiment. Thethickness of glass 1 is determined in large measure by the need toresist hail damage. Four mm thick back-silvered meter-sized panels onfour support pads used in trough thermal systems have given satisfactoryresults in practice. For the larger reflector dishes 1 of thisinvention, glass reflectors 1 having a thickness of about 4 mm thicknessrequire a larger number of supports, optimally distributed so thatbending under gravity and wind forces does not significantly degrade theoptical figure. Preferably, support points that are part of a large,deep open truss are used for glass reflectors 1, providing maximumoverall stiffness for minimum mass and material cost.

A preferred integrated reflector design, shown in FIG. 21, comprises asubstantially square, back-silvered glass reflector 1 and supportingreflector framework. FIG. 22 is a detailed perspective view to show thereflector framework or truss 95 with the glass reflector 1 removed,revealing the sixteen reflector support points 86, 87 & 88 cradling theglass reflector 1. In plan view FIG. 23, these form a roughly squareCartesian grid in a four-by-four pattern. The reflector truss 95 isdesigned for attachment at its base 96 to the four lower corner points11 of a unit reflector cell 34. The four base struts 10 of the unit cell34 combine with two base diagonals 80 that meet at a base center 81, toform four triangles in base plane. Three angled struts 82 are erectedfrom the corners of each said triangle to form four tetrahedra whoseupper nodes 83 are main nodes forming a cradle for the glass reflector1. The five other main upper nodes 84 and 85 are formed at the cornersof two planar frameworks extending up from the base diagonals 80 andsharing the common central member 97 connecting 81 and 84, perpendicularto the base plane. These two orthogonal frameworks are braced by fourdiagonal members 98 connecting the base center 81 to upper nodes 85.FIG. 23 is a plan view detailing just the upper structure of truck beamsand mirror supports 86, 87 & 88. The sixteen cradle support pointscomprise four inner supports 86, eight intermediate supports 87 and fourouter corner supports 88. The inner four and outer four cradle points,86 and 88 respectively, lie on cradle struts 89 extending through nodes84, 85 and 90. The nodes 83 and 90 at the ends of struts 91 carry theintermediate supports 87.

FIG. 24 shows the deflection of a glass reflector 1 supported by thepreferred framework illustrated in FIG. 21, FIG. 22 and FIG. 23, ascalculated by finite element analysis. Deflections were calculated for a3.3 m square paraboloidal reflector 1 with a 1.7 m focal length. Theglass 1 is 4 mm thick and has edges rolled back through 90 degrees witha 25 mm radius. The supporting truss was modeled with a central depth of0.6 m, with struts made from round steel tubes of two-inch diameter and0.015 inch wall thickness. Other cross sections could work as well,provided their cross sectional area and moment of inertia are similar.Referring to FIG. 24, the truss weight for the calculated exampleaverages 3.5 kg per square meter of glass supported. This areal densityof steel is significantly less than the glass areal density of 10.1kg/m² for 4 mm thickness. The average weight for a complete 3 mreflector assembly is thus 13.6 kg/m² for 4 mm thick glass. Thedeflection under the axial gravity load (or equivalently a 30 mph normalwind load) calculated for this example is illustrated in FIG. 24. Thecontour map across the surface shows the change in slope angle caused bythe loading. FIG. 24 shows contour lines 92 that have slope deviationsof 0.05 radians. The deviations in slope, caused largely by sheardeflections in the vicinity of the support pads, over most of thesurface amount to less than 0.1 milliradians, the level marked bycontour B. Contour A corresponds to slope deviation of 0.05 radians.Averaged over the entire surface, 90% of collimated light reflected bythe dish falls within an image of diameter 0.2 degrees, while all thelight lies within a 0.4 degree diameter.

A Preferred Method of Manufacture for a Large, Monolithic and DeeplyDished Glass Reflector

A preferred sequence of process operations to produce a concavemonolithic reflector 1 by shaping an initially flat sheet of glass isillustrated schematically in sequence in FIG. 25, FIG. 26, FIG. 27, FIG.28, FIG. 29 and FIG. 30. The process operations are also illustrated inperspective in the sequence of FIG. 31, FIG. 32, FIG. 33 and FIG. 34.FIG. 25 shows a sheet of glass 200, which is flat at the start of theprocess. FIG. 26 and FIG. 31 show the flat sheet 200 after placementabove a mold 201. A mold 201 has a substantially concave surface 204 androlled edges 203. The glass sheet 200 is supported initially above themold 201 from its corners, which are grasped temporarily by hinged,L-shaped supports 202. FIG. 27 and FIG. 32 illustrate the beginning of asoftening and stretching step. The temperature of the sheet of glass 214is increased preferably by radiative heating from a hot surface 207above the sheet of glass 214. Preferably the radiative surface 207 issubstantially continuous and has high emissivity. Preferably theradiation temperature seen by the glass 214 can be rapidly increased ordecreased, either by using a variable temperature radiator 207 of lowthermal inertia, or by providing relative motion between the mold 201and radiating surfaces 207 of different temperatures. Preferably thetemperature can also be varied as a function of spatial position acrossthe mold 201, for example, a higher temperature is provided around theedges 208 of the radiating surface 207 as compared to temperature of thecentral region of the radiating surface 207, in order to make the edges211 of the glass sheet 214 softer than the center of the glass sheet214. While gravitational force may be used exclusively to drive theshaping of the glass sheet 200 into a concave dish 212, the process ofconforming the sheet of glass 200 to the shape 212 of the mold 201 maybe accelerated by pressure, for example by hot gas jets directed fromabove, or by a pressure differential created in another fashion, such aswith a vacuum pump.

When the glass sheet 212 has sagged enough to make extensive contactalong the edges or sides 203 of the mold 201, as shown in FIG. 33, andthe glass 212 having a shape illustrated in FIG. 28, the corner supports202 may be removed, as shown in FIG. 34. At this stage, edge-shapingtools 210 may be brought into mechanical contact with the outer edges211 of the glass sheet 212 to press and curve the glass edges 211 to adesired rolled shape, as shown in FIG. 28. Also, once the glass sheet214 has conformed and sealed against the curved perimeter 203 of themold 201 as shown in FIG. 27, it may be desirable to hasten or otherwisecontrol shaping the sagging glass sheet 214 into a dish shape 212 byreducing the air pressure below the mold 201. For this purpose, the mold201 may be constructed as shown in FIG. 27, with a vacuum manifold 205beneath, and having holes 206 in the surface in fluid communication withthe manifold 205. The shaping process may then be accelerated byreducing the pressure in the manifold 205 by drawing air down throughthe holes 206 and out through the vent 209 by pumping.

Once the glass 212 has made contact across the body of the mold 201 andits edges 203, thus taking on the mold shape, rapid cooling is started.FIG. 29 illustrates the process, accomplished by radiation from theupper surface of the glass 212 into an absorbent, black surface 213, nowmuch colder than the surface of the glass 212. Forced convection of coolair across the surface of the glass 212 may also be used to speed thecooling step. Once the glass 212 has cooled sufficiently to be stiffenough for short-term handling, it is preferably removed from the mold201 as shown in FIG. 30 for annealing and cooling, leaving the mold 201clear for the next piece of glass 200 to be shaped. The glass piece 212may then be cooled to room temperature at an accelerated rate by forcedair convection.

A mold 201 may be constructed with an underlying structural frameworkshown in plan view in FIG. 35, with a mosaic overlay of precisionmachined plates which make the shaping surface as shown in theperspective cutaway view of FIG. 36. A preferred material for the mold201 is AISI 304 stainless steel. An egg-crate style structural framework215 for a square mold 201, illustrated in FIG. 35, is made from aslotted and interlocking array of identically shaped plates 215 weldedtogether at junction nodes for strength and rigidity. The mold perimeter203, over which the edges of the glass sheet 200 will be rolled, ispreferably made from round tube and welded to the structural framework215. The tube radius is chosen to be the radius of the rolled edge 211to be molded. The perimeter tube 203 may have square corners as shown orclipped corners to produce a glass reflector 1 as shown in FIG. 4.Referring to FIG. 35 and FIG. 36, the overlay molding surface 217 may beassembled from a plurality of separately machined and strain-relievedsegments 217, rigidly attached to the supporting egg-crate structure215. Alternatively, a mold face may be formed in one piece from a spunor hydroformed monolithic dish, machined as a single piece. This piecemay be strain-relived and re-machined as required, before mounting on asupporting framework 215 like that shown in FIG. 35. The segments 217 orthe monolithic single piece of the mold dish may be attached to theframework 215 by supports that take the form of positioning devices (notillustrated). These devices are used to bring the segments intoalignment, with the aid of a metrology device such as laser tracker, notshown.

FIG. 37, FIG. 38, FIG. 39 and FIG. 40 together illustrate the action ofthe corner supports 202, which are hinged about a horizontal axis 236perpendicular to the reflector axis, so they can swing down but not moveradially inwards. This method of restraint ensures that as the sheet ofglass 214 softens and sags, the glass 214 will stretch, thus avoidingthe tendency of a simply-supported sheet 200 to wrinkle by compressivehoop buckling strains when sagged into a deep dish 212. Details of thecorner supports 202 and their operation may be described with referenceto FIG. 37, FIG. 38, FIG. 39 and FIG. 40, all of which show verticalcross sections taken along a diagonal through the supports. FIG. 37shows the corner support 202 prior to softening of the glass sheet 200.The support comprises a bracket attached to the mold 201 and a hinged,L-shaped piece 238 that grips a corner of the glass 200 and turns abouta hinge 236. FIG. 37 shows a substantially flat sheet of glass 200 inposition in the hinged piece 238. FIG. 38 illustrates the support 202when the glass 214 is partially sagged and stretched. FIG. 39 shows theglass 212 when fully sagged and stretched and touching the full body ofthe mold 201, with the support 202 still in place. FIG. 40 shows theconforming glass 212 following the removal of the gripping L-shapedpiece 238 of the support 202. The softened edge 219 of the glass 212 isbeginning to bend over the curved edge 203 of the mold 201.

The operation of the edge rolling tool 210 is shown in perspective viewsin the sequence of FIG. 41, FIG. 42, FIG. 43 and FIG. 44. FIG. 41 showsa section cutaway of a mold plate segment 217 with its tube perimeter203, and a cutaway of the edge shaping tool 210 poised above the mold201. FIG. 40 and FIG. 42 show a cutout of a slumped glass sheet 212,still hot and soft, with a partially-slumped edge 219 formed by saggingunder its own weight. FIG. 43 shows the edge tool 210 making intimatepressing contact with the glass 212 in order to form the rolled edge211. FIG. 44 shows the finished rolled edge 211 of the glass 212. Thespecular quality of the rolled glass edge 211 is not important and sothe shaping tool may have a conventional, smooth machined finish.

In one preferred implementation, illustrated in FIG. 45, a mold 201 ismade with a grooved surface profile 218, in order to minimize damage tothe specular back surface of the glass 212 by contact with the moldsurface. The surface of the molding plate 217 is final-machined bypasses of a ball end mill tool to create adjacent concave grooves 220that intersect in cusps 221. The cusp surface profile 222 defines theshape that the glass 212 will adopt. Typically the mold 201 will bemachined to within +/−10 microns of the local shape of the reflector 1.During shaping, as shown in FIG. 46, the softened glass sheet 212contacts the line of cusps 221, and bridges between them, taking itsoverall shape as defined by the surface profile at the peak of thecusps. Since the natural fidelity of replication is excellent, theslumped glass 212 will adopt the shape of the mold 201 on spatial scaleslarger than the groove spacing, and will preserve the natural smoothnessand specular reflection of virgin float glass 200 on smaller scales.

In detail view of the glass-mold contact, shown in FIG. 47, the glass212 droops slightly 224 between cusps 221, where the droop depth dependson the contact dwell time and the glass viscosity at the slumptemperature. The shaping process is optimized such that the dished shape212 is largely formed by free stretching and sagging before the glass212 lightly touches and comes to rest on the cusps of the mold 211.Rapid cooling to prevent continued drooping between the cusps is startedas soon as contact is made across the mold 201. In this way, marring ofthe glass 212 is localized to narrow lines of small dimples along thecusps 221, and wear of the mold 201 is minimized. For this process, thenet solar collecting area lost to scatter from contact damage at cusps221 and surface slope errors from resulting dimples in the glass 212 isfound to average no more than 0.25% of the total glass surface area, andin practice the slope angles for drooping between grooves 221 having aspacing in the range of one to three centimeters are typically less than0.1 degrees rms.

A further advantage of the cusped mold surface 218 of this invention isthat direct thermal conduction from the mold 201 to the glass 212 duringthe thermal cycle of shaping is minimized, because the area of contactline 221 is so small. Minimal heating of the mold 201 insures that shapewarpage caused by temperature gradients is minimized. Another advantageis that the concave valleys 220 also provide regions 220 that gathersmall particulate contamination, keeping small particles sub-flush tothe cusp 221 during slump and thereby reducing dimples in the glass 212that might otherwise have been caused by protruding particles on thesurface 220 of the mold 201.

In a second preferred alternative to minimize contact damage, cushionsof air are used to prevent any direct contact between the glass 212 andthe mold 201. A preferred implementation has a double manifold below themold surface. One manifold is pressurized while the other manifold isevacuated. The pressurized manifold supplies air to the air cushions,while the evacuated manifold carries away the pressurizing air andensures overall downward pressure. In this alternative embodiment, thecushions are spaced by one to four centimeters, with evacuating holesbetween.

Heat transfer to the slump mold 201 is preferably minimized during theglass shaping thermal cycle, to minimize thermal cycling and to hold thetemperature on the mold 201 well below the critical oxide-scale limit of800 C for temperature-cycling of AISI 304 stainless steel. The internaltemperature gradient within molding plate segments and across the mold201 will preferably be minimized to reduce warpage tendency, so as toreplicate glass 212 having a proper and consistent shape. The mold 201is preferably coated for high reflectivity to the thermal radiationtransmitted and emitted by the glass 212, in order to minimizeabsorption. Residual bowing of the mold faceplates or facesheet isrestrained by fasteners located across the rigid framework. After aglass sheet 200 has been shaped into the desired dish configuration 212,the mold face will be brought into thermal equilibrium and restored tothe preferred starting temperature by forced air convection before beingused to shape the next sheet of glass 200. The underlying structureremains effectively athermal and undergoes no significant thermaldistortion during the small radiation exposure of shaping.

A preferred method for increasing mold reflectivity is to coat thesurface with 10-50 microns of an abrasion resistant, highly reflective,uniformly thick, and tenacious dense metallic film that maintains highreflectivity after several months of use at temperatures up to 400 C.Coating methods such as electrolytic or electroless plating, sputtering,plasma and thermal spraying, vapor deposition and combinations of thesemethods are preferred. Preferred plating metals include chromium, nickeland noble metals such gold, platinum, rhodium and their refractoryalloys such as nickel-chromium, nickel-doped phosphorous,platinum-rhodium, and platinum-iridium.

In order to supply very large numbers of glass reflectors 1 at lowestpossible cost, the glass shaping described above and a back-silveringprocess will preferably be incorporated into a float glass factory,whose entire glass output will be in the form of solar reflectors 1. Anautomated continuous process according to this invention, to provide forshaping process at the speed required for integration into a float glassfactory, is shown schematically in FIG. 48. Float glass 231 is drawnfrom the melting furnace and tin float bath 230 where the glass emergesas a ribbon 231 at approximately 600 C onto a conveyor system 232. Herethe float glass 231 is cut into square sheets 200 while still hot. Onepreferred method to cut hot glass 231 into sheets 200 before the shapingstep in the production line is by scoring and breaking, which is theoldest and least expensive of all cutting methods. This requires theglass 231 to be cooled either globally or locally in the region to becut to its brittle temperature range, below approximately 350 C. Analternative preferred method, which avoids the need to cool glass 231just to cut it, is laser cutting. Suitable high energy laser systems forhigh-speed cutting of a hot glass ribbon 231 are disclosed in U.S. Pat.No. 3,885,943, to Chui, in U.S. Pat. No. 3,930,825, to Chui, and in U.S.Pat. No. 3,934,995, to French, all of which are incorporated herein byreference.

The substantially flat cut sheets 200, cut from glass ribbon 231 using acutting apparatus 233 (for example laser cutting device or scoring andbreaking device), are cooled to a stiffness that allows handling fromcorners by corner clamps 202. Once gripped by the clamps 202, the sheet200 is positioned above a slump mold 201. The mold 201 and sheet 200then enter the shaping tunnel 207 where intense radiant energy from anextended blackbody at temperatures in the range of 800 C to 1200 Cstrikes the top surface of the glass 200 and rapidly boosts it tosagging temperature resulting in a sagging glass sheet 214. Shaping ofthe hot glass by the steps already described above takes place withinthe hot tunnel. Sagging, stretching and molding of the glass to shape212 is accomplished quickly by gravity, accelerated if desired byapplication of pressure. Edge-shaping tools 210 form the rolled edges211, including at the corners after removal of the sheet's corner clamps202. After shaping is complete, the glass 212 enters a cooling tunnel213, where it is quickly cooled by both convection and radiating heataway to a cold surface that absorbs radiation. When cooled enough torecover sufficient rigidity to hold its shape, the formed glass 212 isremoved from its mold 201 and transferred to the conveyor line 213,which moves the glass 212 along for annealing and cooling. Molds 201 arecycled 235 and re-introduced to the production line.

At the far downstream end of the production line 234, silver is applied,preferably by vacuum plasma assisted processes similar to that used todeposit silver and dielectric layers on float glass in the manufactureof low E architectural windows. Following conventional minor fabricationpractice, the silver surface and leading edges of the glass 212 areover-coated with a copper film oxidation barrier, also preferably byplasma deposition. Finally, the copper film and mirror edges arepowder-coated with a weather-resistant polymer seal. The finished glassreflectors 1 are stackable, and may be stacked and shipped forintegration with trackers 107 and energy converters 4, and forinstallation at solar farm sites.

In an alternative embodiment of the float glass, the glass ribbon 231,after being cut by the cutting apparatus 233, is fed into two or moreshaping lines, to ensure continual production if one production linesuffers breakdown or is slowed in its operation. In addition, a separatebranch may be provided to cool and package flat glass sheets 200 in caseall shaping lines break down. Glass sheets 200 may be stored for shapingoffline.

The number of molds 201 and the length of the production line needed tosupport continuous production at the full rate of output available froma float glass factory, depend on the cycle time for each mold 201. Forexample, in a process where this cycle time is two hundred seconds and anew sheet 200 to be processed emerges from the tin float bath 230 everyten seconds, then twenty molds 201 must be in use at once. Themanufacturing process of this invention is designed to minimize the moldcycle time, and thus minimize the number of molds 201 in use on theproduction line and the size of the facility. The mold cycle time is setlargely by thermal equilibration and heat transfer factors. For example,thermal equilibration and heat transfer times involved in heating andsoftening the glass 214, and the time required for stretching andthermal equilibration and cooling down to make the glass 212 rigid so itcan be released from the mold 201, will be factors affecting mold cycletime.

The temperature to be reached for shaping glass sheets 200 is determinedby the need to accomplish sagging and stretching in a relatively shorttime. To determine the speed of glass stretching into the dish shape 212under gravity, the gravitational potential energy lost with sagging isequated with that dissipated by stretching the viscous glass. Forexample, for a square sheet 200 clamped at its corners 202 and saggingunder its own weight by membrane-type stretching of the glass, we showthat the saggital depth “s” at the center increases with the cube rootof time “t” and viscosity “η” approximately ass=(cgρx ⁴ t/η)^(1/3),  (1)where “c” is a numerical constant (approximately 0.044), “g” is theacceleration due to gravity, “ρ” is the glass density, “x” is theaverage span length between mold supports, and “i” is the shearviscosity. The shear viscosity of typical commercial soda-lime glass inthe temperature range 500-750 C varies with absolute temperature T asη=η_(o) exp(B/T), where η_(o)=1.6×10⁻¹⁸ poise and B=5.8×10⁴ K. For thepreferred shaping temperature range between 650 C and 750 C, theviscosity drops from 4.3×10⁹ poise (650 C) to 9×10⁶ poise (750 C).

As a specific dimensioned example of a preferred reflector 3.1 m square,the time to sag to a preferred depth s=0.795 m at the center from theabove equation is approximately 20 sec at 710 C, and varies by a factorof two for each ten degree change of temperature. Thus the uppertemperature for the process will be in the region of 710 C. At the otherextreme, the distortion on handling for one minute is reduced to anegligible level at temperatures less than 500 C.

In the process described in this invention, heating and cooling betweenthese temperatures is accomplished in large measure by radiationtransfer. For heating, radiation from a substantially continuousblackbody at 800-1200 C is preferred, since the thermal radiation atthese temperatures is largely absorbed by the glass. Thus a 4 mm thickglass sheet 200 may be heated from room temperature to an averagetemperature of 710 C in fifty seconds by radiation on one side from a900 C blackbody, and in twenty seconds from a 1200 C blackbody. If theglass 200 is hotter at the start of heating, less time is needed.

After rapid radiative heating, the glass 214 is much hotter and thusless viscous on the heated side. The time constant for equilibration byconduction across a sheet of soda-lime glass is approximately nineteenseconds for 3 mm thickness, thirty-four seconds for 4 mm, andfifty-three seconds for 5 mm. Some degree of equilibration is preferredas the glass 212 takes up the mold shape and the edges 211 are rolled.When shaping is complete, the glass 212 will be cooled from about 710 Cto about 500 C by radiation into a cool absorbing surface 213, takingabout fifty seconds for a 4 mm sheet, and followed by around thirtyseconds for equilibration at 500 C before removal.

A mold cycle for shaping 4 mm thick glass according to the aboveoperations taken in sequence may for example include fifty seconds forheating by 900 C radiation, twenty seconds for sagging, thirty secondsfor equilibration, settling on the mold and edge shaping, fifty secondsfor radiative cooling, thirty seconds for equilibration before removal,and sixty seconds to return the mold 201 for reloading, for a total of240 seconds. For a ten second glass cycle time, twenty-four molds are inuse at once, and the thermal processing part, hot shaping and cooling,will take 180 seconds. For a mold-to-mold separation of four meters, andthus a line speed of 0.4 m/sec, the shaping line length will be aroundseventy-two meters.

During the development of the above high volume process, glassreflectors 1 may be manufactured off-line at a slower rate from stackedcold glass sheets 200. The initial stages are different, and require thecold glass sheets 200 to be handled safely and brought to the slump mold201 without breakage. The cold sheets 200 are picked from a verticalstack with a multi-cup vacuum-lift fixture that holds the glass 200everywhere flat within a few mm and in a low-stress state. The sheet 200will then be turned horizontal, and gripped from the corners usingcorner grips 202 on a handling frame. The vacuum handling fixture isreleased to unload the sheet 200 and the glass relaxes slightly in itsgripped state. The sheet loading procedure and non-translating cornergrips 202 keep the maximum tensile stress below the 3.5 MPa safetylimit.

Glass sheets 200 that are loaded in this manner into handling frameswill be conveyed through a conventional pre-heating tunnel to a peaktemperature of approximately 500 C. From there on, a glass sheet 200using its handling frames is mated with a shaping mold 201 as in FIG. 26and the process continues as previously described.

Those skilled in the art, after having the benefit of this disclosure,will appreciate that modifications and changes may be made to theembodiments described herein. Changes may be made in the steps of themanufacturing process, additional steps may be added, differentmaterials may be substituted, and equivalent features used, all withoutdeparting from the scope and spirit of the invention. This disclosurehas set forth certain presently preferred embodiments and examples only,and no attempt has been made to describe every variation and embodimentthat is encompassed within the scope of the present invention. The scopeof the invention is therefore defined by the claims appended hereto, andis not limited to the specific examples set forth in the abovedescription.

What is claimed is:
 1. An apparatus for generating electricity fromsolar radiation, having a dish-shaped reflector, said dish-shapedreflector being adapted to concentrate solar radiation at a point offocus, and a substantially spherical lens positioned near the point offocus of said dish-shaped reflector, and further having a plurality ofphotovoltaic cells positioned at a receiving surface located in anoptical path beyond said substantially spherical lens, saidsubstantially spherical lens being positioned in the optical path tospread the solar radiation over said plurality of photovoltaic cells togenerate electricity when said photovoltaic cells are illuminated withsolar radiation, wherein said dish-shaped reflector is made by a processcomprising the steps of: heating a sheet of glass positioned over a moldhaving a plurality of adjacent concave grooves that intersect in aplurality of cusps, minimizing direct thermal conduction from the moldto the glass; forming the glass sheet to the plurality of cusps bysagging and stretching the glass sheet until the glass sheet touches theplurality of cusps on the mold and droops between the plurality of cuspson the mold without significantly degrading the optical smoothness ofeither the front or back surface of the glass; and, creating areflective surface on the glass to form at least a portion of areflector dish operative to reflect solar energy to a focal point forconversion of said solar energy into useable energy; wherein saiddish-shaped reflector made by said process has dimples in the sheet ofglass corresponding to said cusps and droops in the sheet of glassbetween said dimples.
 2. The apparatus made by the process of claim 1,wherein said process further comprises the step of cooling the sheet ofglass after it touches said plurality of cusps on the mold to preventcontinued drooping of the sheet of glass between the cusps of the mold.3. The apparatus of claim 1, wherein the mold has been coated withchromium.
 4. The apparatus of claim 1, wherein the mold has been coatedwith nickel.
 5. The apparatus of claim 1, wherein the mold has beencoated with a nickel-chromium alloy.
 6. The apparatus of claim 1,wherein a distance between the dimples is between one to threecentimeters.
 7. The apparatus of claim 1, wherein a slope angle of thedroops is less than 0.1 degrees rms.
 8. The apparatus of claim 1,wherein a back surface of the dish-shaped reflector is curved betweensaid dimples.
 9. An apparatus for generating electricity from solarradiation having a plurality of large dish-shaped reflectors each havingan associated conversion unit, each dish-shaped reflector having an axisof symmetry, each dish-shaped reflector being shaped to reflect solarradiation to a focus, each conversion unit incorporating a substantiallyspherical lens positioned near the focus of a corresponding dish-shapedreflector to spread the solar radiation over a plurality of photovoltaiccells, each conversion unit having a plurality of photovoltaic cellspositioned in a concave array located in an optical path beyond thesubstantially spherical lens to generate electricity when thephotovoltaic cells are illuminated with solar radiation, said apparatusbeing supported by a moveable structure holding the dish-shapedreflectors and associated conversion units in a rigid array with saidaxes of symmetry in coalignment and each conversion unit positionedabove the corresponding dish-shaped reflector associated with saidconversion unit and centered on said axes of symmetry, said moveablestructure comprising a plurality of struts forming a 3-dimensional trussstructure in which struts linking the dish-shaped reflectors to theconversion units additionally provide depth to the structure and providestiffness, said moveable structure orientable to face the sun in orderto deliver solar radiation to said conversion units, wherein eachdish-shaped reflector is made by a process comprising the steps of:heating a glass sheet positioned over a mold having a plurality of cuspsforming an operative mold surface that minimizes direct thermalconduction from the mold to the glass; sagging and stretching the glasssheet until it touches and then substantially conforms to the operativemold surface and droops between the plurality of cusps on the mold,without significantly degrading the optical smoothness of either thefront or back surface of the glass; and creating a reflective surface onthe glass to form at least a portion of a reflector dish operative toreflect solar energy to a focal point; wherein each dish-shapedreflector made by said process has dimples in the sheet of glasscorresponding to said cusps and droops in the sheet of glass betweensaid dimples.
 10. The apparatus made by the process of claim 9, whereinsaid process further comprises the step of cooling the glass sheet afterit touches the operative mold surface to prevent continued drooping ofthe glass sheet between the cusps of the mold.
 11. The apparatus ofclaim 9, wherein the mold has been coated with chromium.
 12. Theapparatus of claim 9, wherein the mold has been coated with nickel. 13.The apparatus of claim 9, wherein the mold has been coated with anickel-chromium alloy.